This invention relates to an annular elastomeric track for use in vehicles for civil engineering, construction works and agricultural works.
Molded rubber tracks are in many cases being substituted for conventional metal tracks. Rubber tracks offer better maneuverability, better ride quality in rough fields, better flotation in wet areas, improved side hill stability, excellent traction, low maintenance and versatility compared to steel tracks.
Additionally, rubber tracks are replacing conventional rubber tires on tractors and other agricultural vehicles such as combines, trenchers, snow removers, spreaders, sprayers, wagons and carts, since rubber tracks are more friendly to agricultural fields, offer better flotation and less compaction than rubber tires, resulting in better crop yield. The use of rubber tractor tracks permits farmers to get in and out of fields earlier in the planting season and plant more crops as compared to rubber tire-equipped agricultural vehicles.
In civil engineering applications tracks are employed on many construction equipment type vehicles such as road pavers and the like. These tracks are generally made from steel or steel with urethane or rubber pads. These tracks require much maintenance and in the case of steel tracks the damage to paved surfaces is a serious problem. Also, these construction type tracks can be placed over tires for skid steer equipment. A new advance in this area is the use of an all rubber steel cord track. The Goodyear Tire and Rubber Company provides such a track under the trademark TRACKMAN(copyright), and the Bridgestone/Firestone Company provides a similar track under the name Firetrax(copyright). These tracks have rubber lugs that extend across the entire width of the track.
Rubber tracks are defined by an endless rubber belt or band reinforced with continuous flexible steel cables bonded into the rubber. Presently, a complete rubber track is molded flat in multiple sections, which are sequentially then bonded together during the curing process. Alternatively an endless rubber track can be made in annular form wherein the track is made from an uncured belt and a plurality of drive lugs wherein the drive lugs are urged into cavities formed in inner segments of a molding press, partially final formed and pressed onto the inner periphery of the belt, the belts and drive lugs are positioned in the molding press to be cured and molded together while treads are formed on the outer periphery of the belt. The drive lugs are finally formed and the completed belt is cooled and removed from the molding press forming an annular track as described in U.S. Pat. No. 6,051,178 issued Apr. 18, 2000, to the assignee, The Goodyear Tire and Rubber Company.
In each of the methods of manufacture the outer periphery has a plurality of spaced tread lugs. Generally in the prior farm tire art these tread lugs extend from each lateral edge of the track towards the center of the track terminating at the central location or mid-point of the track. These lugs generally take the appearance of agricultural tire lugs with a generally inclined but slightly more transverse extending inclination relative to the direction of travel. Additionally, these lugs can be staggered or circumferentially offset from one side of the track to the other. This helps insure that the track has sufficient bending moment as the track traverses over the drive wheels and guide wheels at each extremity of the elliptical shape track as it traverses in use. If the tread lugs extend from one side of the tracks"" lateral edge to the opposite side, then it is believed that the lugs should be substantially perpendicular to the direction of the circumferential links of the track in order for the lugs to achieve appropriate bending moment in flexibility as it rotates around the drive wheels. For this reason, it is felt necessary to provide the tracks with two sets of lugs that act somewhat independently relative to the lugs set of the opposite side of the track so that the track itself can bend in a rather flexible nature. Absent this bending, severe loads result in the belt reinforcing structure causing potential separations and other defects in the track itself as has been observed in some of the prior art rubber construction tracks previously mentioned.
The belt reinforcing structure of a track has at least one primary cable for reinforcing the track in the circumferential direction and usually at least an additional two layers of crossed angled cords which provide lateral stability. Each layer of cords is encapsulated in a layer of rubber. The entire belt structure must be very flexible and therefore is designed to be thin in cross-sectional thickness. This thin structure is sometimes cut by debris trapped internally or cut by simply driving over debris such as concrete rubble, steel or trash found at typical construction sites or in the fields.
Once the belt is cut the reinforcing cords are exposed to moisture which can quickly oxidize the steel cords. Typically a thin brass coating is applied to the steel to improve adhesion to the rubber. The reaction of brass coated steel to oxidation is such that the steel preferentially corrodes relative to the brass coating. The brass plating when combined with the steel gives good to excellent rubber adhesion. A secondary failure of a rubber track occurs if the cords separate locally from the rubber. When this situation occurs, the reinforcement cords of the belt progressively separate from the track ultimately making the track useless.
To extend track life great efforts are expended in developing new tread compounds, belt rubbers, and guide lug compounds.
One way to increase track durability would be to improve the corrosion resistance of the belt reinforcing structure while maintaining high cord to rubber adhesion. The following invention discloses a novel construction to achieve this result.
An endless elastomeric track has guide lugs on the inner circumference and rubber tread lugs on the outer circumference and a thin band continuously extending in the circumferential direction. The thin band is circumferentially reinforced by substantially inextensible cords. Preferably the cords are steel reinforced materials.
The track has at least one primary cable of galvanized steel circumferentially reinforcing the thin band. The at least one primary cable is oriented generally circumferentially and is preferably helically wound forming one reinforced layer extending from a first end on a first side of the track to a second end on the opposite end of the track. The at least one primary cable has a diameter of between 2.0 mm and 14.0 mm, preferably about 5.3 mm. The at least one primary cable is galvanized steel and in the preferred embodiment has a wire bundle construction having ((3xc3x970.35 mm)+(6xc3x970.63 mm)+6xc3x97(0.63 mm+6xc3x970.57 mm)).
The primary cable 59 is encapsulated in a rubber 22 having a rubber mixture composition comprising of the following, based upon 100 parts by weight elastomers (phr)
(A) elastomers comprised of 20-100 phr isoprene rubber and corresponding zero to 80 phr of styrene/butadiene rubber;
(B) 30 to 70 phr of reinforcing filler selected from rubber reinforcing carbon black and silica comprised of 20 to 70 phr of said carbon black and from zero to 40 phr silica;
(C) Calcined litharge 2 to 5 phr;
(D) Sulfur from 2 to 5 phr; and
(E) Zinc oxide from 3 to 9 phr.
Optionally the composition may further include a reinforcing resin system such as resorcinol and HMMM in amounts of about 2.2 phr and 3.2 phr, respectively. Alternatively, the calcined litharge can be substituted with cobalt salts in the range of 0.5 phr to 3 phr.